Ion exchange agglomerates have been prepared in which insoluble substrate particles of synthetic resin having ion exchange sites on their surface are coated with insoluble synthetic resin coating particles having ion exchange sites which attract the sites of the substrate and irreversably attach to the substrate. Such compositions are used for removal and separation of ions, particularly inorganic anions, and are especially useful in liquid ion exchange chromatography. In a specific example, the ion exchange composition includes surface sulfonated resin beads of styrene-divinylbenzene copolymer and a monolayer of quaternary ammonium styrene-divinylbenzene copolymer resin coating particles in the hydroxy form attached to the surface of the beads. A chromatographic separation of anions may be effected in a bed of such particles by elution with an aqueous sodium hydroxide solution.
A technique which has been employed to form the aforementioned agglomerates is as follows. A liquid suspension of the relatively large beads in a nonsolvent slurry is deposited in an ion exchange column. Then, a dispersion of the coating particles is passed through the column. Completion of formation of the monolayer is indicated by breakthrough of the coating particles in the column effluent stream. In this in situ process, the column is then employed in that form for liquid chromatography.
There are a number of disadvantages to the above process. As the columns are formed in situ, it is apparent that those areas where the resin beads contact each other are blocked from exposure to the coating particles. This leads to bald spots on the agglomerated resin beads. Any rotation of the agglomerate resin beads leads to a loss of liquid chromatography capacity and consequent loss of resolution. This is illustrated by very poor separation between bromide and nitrate ions and relatively poor separation between nitrite ions and chloride ions. In addition, it has been found that acceptable columns for use in this type of separation can only be formed in about 25 to 30% of the columns packed. Furthermore, a portion of the more expensive coating particles is lost as breakthrough from the column is the indicator used for completion of the reaction. Finally, a common form of the active sites of a coating particles is the carbonate form. For use in this process, the column must be equilibrated with carbonate, e.g., sodium bicarbonate or sodium carbonate, an additional step.